It should be noted that any discussion of the prior art throughout the specification is not an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Personal flight devices were developed in the 1960's and 1970's, but were essentially rocket based devices (jet belts) which gave extremely short flight times (typically about 26 seconds) and were difficult to control. Further, these devices were fuelled by rocket fuel which is intrinsically dangerous.
In a related type of device fuel is fed to a catalyst bed to produce hot gas which is distributed between two spaced counter-rotating turbines to generate vertical thrust from the turbine exhausts. The device is steered by shrouds or deflector rings pivoted to the base of each turbine.
More recently, a number of personal flight devices have been proposed which use ducted fans as their means of propulsion:—for example a device which includes a pair of ducted fans and which is steered by tilting the ducted fans relative to the pilot.
A broadly similar type of design has been proposed, using shrouded propellers rather than ducted fans.
As the terms are used in this specification, the distinction between a ‘shrouded propeller’ and a ‘ducted fan’ essentially is that a ducted fan comprises one or more rotating aerofoils or fans (single or multi-bladed) mounted completely within a duct; the duct increases the overall efficiency of the fan. A shrouded propeller comprises a rotating aerofoil or propeller which is surrounded by a shroud, the only function of the shroud being to protect the propeller—the shroud does not appreciably affect the efficiency of the propeller.
A further proposal describes a device which includes a pair of ducted fans. In this device, the fans are rigidly secured to the supporting harness, and steering is achieved by control flaps mounted adjacent to the outlet of each fan. However, in this design the two fans are contra-rotating and thus have to be driven via a gearbox using a relatively complex rigid transmission system; this adds considerable weight to the device.
Other prior art proposals have been for single shrouded propellers or ducted fans. Single propeller/fan devices have the drawback that the propeller/fan has to be extremely large to provide adequate lift, and to balance the devices the pilot generally has the propeller/fan above or below him, which is far from ideal from a safety point of view. A further drawback is that the thrust from a single propeller/fan has to be split in some way to give steering and this tends to result in high frictional losses in the steering tubes or ducts.